In mouse embryos at various stages of development mitoses are always found in the choroid plexus in both the epithelium and the connective tissue. The presence of mitoses here is in contrast to the apparent lack of mitoses in the choroid plexus of human embryos as previously described by several authors. The location of epithelial mitoses in different parts of the choroid plexus is in principle the same in untreated and colcemid-treated mouse embryos, even though the number of visible mitoses is increased up to 20 times by using colcemid. In the plexus of the lateral ventricles a broad peripheral zone of the surface nearest the free border is always completely without epithelial mitoses (on average, 40-50% of the surface area). In the 3rd and 4th ventricle plexuses only a few of the mitoses in the epithelium are found peripherally on villi (on average, 5-7%). Thus the epithelial cells seem to be formed in certain zones near the root and not diffusedly. In the choroid plexus connective tissue, however, mitoses occur everywhere, indicating an interstitial growth. The largest number is found where the connective tissue occurs in the greatest quantity. An average of 28% of mitoses in the connective tissue in the lateral ventricle plexuses are found under the mitosis-free portion of the epithelium.